Engineers are constantly seeking ways to reduce undesirable engine emissions without over reliance upon exhaust after treatment techniques. One strategy is to seek ways to improve performance of fuel injection systems. Over the years, engineers have come to learn that engine emissions can be a significant function of injection timing, the number of injections, injection quantities and rate shapes. However, it is also been observed that an injection strategy at one engine operating condition may decrease emissions at that particular operating condition, but actually produce an excessive amount of undesirable emissions at a different operating condition. Thus, for a fuel injection system to effectively reduce emissions across an engine's operating range, it must have the ability to produce several different rate shapes, have the ability to produce multiple injections and produce injection timings and quantities with relatively high accuracy. Providing a fuel injection system that can perform well with regard to all of these different parameters over an entire engine's operating range has proven to be elusive.
In order to reduce hydrocarbon emissions, the conventional wisdom has been to seek an abrupt end to each injection event. This strategy flows from the conventional wisdom that reducing poorly atomized fuel spray into the combustion space toward the end of an injection event can reduce the production of undesirable hydrocarbon and smoke emissions. In the case of fuel injectors equipped with direct control needle valves, an abrupt end to injection is often accomplished by applying high pressure fluid to the back side of a direct control needle valve member to quickly move it toward a closed position while fuel pressure within the injector is relatively high. Recent data from some directly controlled fuel injection systems appear to show higher hydrocarbon and smoke emissions at certain operating conditions than those typically observed in relation to older systems in which the nozzle is controlled by a simple spring biased needle. In some fuel injection systems, closing the needle valve member at high pressure can also have structural consequences. When a needle is closed at high injection pressures, pressure can spike within the injector, and especially in the relatively sensitive area of the injector tip, exacerbating the structural strength requirements in the tip region of the fuel injector. These pressure spikes can sometimes cause small uncontrolled secondary injections that increase hydrocarbon emissions. In the case of hydraulically actuated fuel injection systems, closing the needle at high pressure can also result in a reduction in efficiency. This occurs when pressurized actuation fluid continues to pour into the fuel injector briefly after the needle has moved to close the nozzle outlet. Ending injection events at high pressure can also exacerbate the already difficult problem of producing small injection quantities, such as precisely controlled small post injection quantities.
One effort to deal with venting pressure at the end of an injection event in order to avoid small uncontrolled secondary injections is disclosed in U.S. Pat. No. 5,682,858 to Chen et al., and entitled Hydraulically-Actuated Fuel Injector With Pressure Spike Relief Valve. In this fuel injection system, closure of the direct control needle valve member occurs before the flow control valve can end supply of high pressure actuation fluid to act on an intensifier piston. This reference teaches the use of a separate pressure relief valve that opens to relieve actuation fluid pressure as the flow control valve is moving from its open position toward its closed position. This relief of actuation fluid pressure in turn relieves the downward force on the intensifier piston/plunger to also relieve fuel pressure to avoid a pressure spike. While this strategy may be effective in reducing undesirable and uncontrolled secondary injections, there still remains room for reducing hydrocarbon emissions from engines using this type of fuel injection system.
The present invention is directed to one or more of the problems set forth above.